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Rheic Remains in Pedras Ruivas

Closing of the Rheic Ocean

The Rheic Remains in Pedras Ruivas Local Natural Monument off­ers various geomorphological interests like the residual and tectonic, and others, like the periglacial, the paleontological, the geocultural, the mineralogical and ecological. This is a key location that preserves essential elements for the interpretation of Rheic’s hydrodynamic, namely the interpretation of sedimentary processes operating at the sandy-muddy bottom of this ancient ocean (ripplemarks and fossilized laminations) and the paleobiodiversity installed (ichnofossils of arenicola – Scholitus sp. and trilobites – Cruziana sp.) (Carvalhido, 2012).

There are some remains related to the way that the Rheic’s closing was processed and evolved (cf. Alcantilado de Montedor Local Natural Monument) such as the overlap of various episodes regarding dominant tangential tectonics, specifically during the phases of hercynic deformation D1 & D3 (Pamplona et al., 2006). These events were responsible for the main geometrical aspects of the meta-sediments, like the domino and sigmoidal structure, and the quartzite folding of the St. Justa Fm. as well as kink foldings of the schists of the Valongo Fm. (sinistral strike-slip fault syn-D1). The dextral strike-slip fault syn-D3 was responsible for the late settlement of veins and dykes by local distention.

In this area there are structures that at the geological timescale, are really recent. Beyond the rock’s flattened tops, remains of pre-existing beaches from the last interglacial period (125 thousand years old), sediments of an ancient lake, which endured in this landscape during around 7 thousand years (from 18000 to 11000 years old), are preserved, filled by the spring fusion waters produced by the melting snow of local hillsides (Carvalhido, 2012).

References:

The Rheic Remains in Pedras Ruivas Local Natural Monument off­ers various geomorphological interests like the residual and tectonic, and others, like the periglacial, the paleontological, the geocultural, the mineralogical and ecological. This is a key location that preserves essential elements for the interpretation of Rheic’s hydrodynamic, namely the interpretation of sedimentary processes operating at the sandy-muddy bottom of this ancient ocean (ripplemarks and fossilized laminations) and the paleobiodiversity installed (ichnofossils of arenicola – Scholitus sp. and trilobites – Cruziana sp.) (Carvalhido, 2012).

There are some remains related to the way that the Rheic’s closing was processed and evolved (cf. Alcantilado de Montedor Local Natural Monument) such as the overlap of various episodes regarding dominant tangential tectonics, specifically during the phases of hercynic deformation D1 & D3 (Pamplona et al., 2006). These events were responsible for the main geometrical aspects of the meta-sediments, like the domino and sigmoidal structure, and the quartzite folding of the St. Justa Fm. as well as kink foldings of the schists of the Valongo Fm. (sinistral strike-slip fault syn-D1). The dextral strike-slip fault syn-D3 was responsible for the late settlement of veins and dykes by local distention.

In this area there are structures that at the geological timescale, are really recent. Beyond the rock’s flattened tops, remains of pre-existing beaches from the last interglacial period (125 thousand years old), sediments of an ancient lake, which endured in this landscape during around 7 thousand years (from 18000 to 11000 years old), are preserved, filled by the spring fusion waters produced by the melting snow of local hillsides (Carvalhido, 2012).

References:

Technical details

Location

This place should be visited with some caution, because depending on the maritime conditions, the geosites may not be visible. Try to visit at low tide and with a calm sea.

Coordinates

Lat: 41.6947628

Long: -8.8500548

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Point of interest

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Technical details

The Iberian Peninsula is part of the internal range of the Varisca Chain that extends for more than 3000 km, from Morocco to the north of Bohemia. It resulted from the approach and collision (starting in the Middle Devonic) of the NW portion of Gondwana (current African territory) and Laurentia - Baltica, separated by Tethis, and whose closing position corresponds to the suture zone marked by the Porto-Tomar shear. The survey of the Varisca Chain took more than 70 million years, having a polyphasic and zonal character depending on the characteristics of the approximation vector of the two supercontinents involved in the structures of the Iberian Massif, allowing the definition of several geotectonic zones based on the different paleogeographic, tectonic, metamorphic and magmatic characteristics, evidenced for each sector (Matte & Ribeiro, 1975; Dias & Ribeiro, 1995). Tectogenesis was accompanied by regional metamorphism and magmatism, which mainly affected internal areas.

Accordingly to Carvalhido (2012, 2014a,b,c) the knowledge of sea level changes is essential for understanding the quaternary evolution of sedimentation environments, since these disturbances influence the characteristics of the processes that determine the supply and distribution of particles.

The Darque Surface has several types of continental and transition environments sediments, which document paleoenvironmental conditions that varied between MIS9 (340ka) and MIS1 (present).

Sedimentary units identified in this natural monument reproduce evidence of the environments at your deposition:

The Cambôa do Marinheiro silts, dated below 20 ka and up to about 13 ka (most recent date obtained) (MIS2), show that sedimentary processes have become essentially dependent on drainage in conditions of greater water scarcity, which indicate the progression and peak of the installation of the cold conditions resulting from the last glaciation. The conditions of diffuse drainage will have contributed to the formation of lacustrine bodies of high confinement, of swampy character, installed in shallow depressions of the Coastal Platform.

The Conglomerates and Sands of Rego de Fontes correspond to the installation of depositional means with some variability of facies, with dating <13 ka (MIS2 / 1). In general, the character of episodic water availability is observed, namely represented in drainage under low granoselective regimes, sometimes torrential (grain-flow to debrisflow), evolving to facies associated with soaked soils responsible for solifluxive processes.


References:

Carvalhido, Ricardo (2012). O Litoral Norte de Portugal (Minho-Neiva): evolução paleoambiental quaternária e proposta de conservação do património geomorfológico. Universidade do Minho.

Carvalhido, R.; Pereira, D. & Cunha, P. (2014a). Depósitos costeiros quaternários do noroeste de Portugal (Minho - Neiva): caracterização datação e interpretação paleoambiental. Comunicações Geológicas (2014) 101, Especial II, 605–609.

Carvalhido, R.; Brilha, J. & Pereira, D. (2014b). Monumentos Naturais Locais de Viana do Castelo: processo de classificação e estratégias de valorização. Comunicações Geológicas (2014) 101, Especial III, 1219–1223.

Carvalhido, R.; Pereira, D; Cunha, P.; Buylaert, J. & Murray, A. (2014c). Characterization and dating of coastal deposits of NW Portugal (Minho-Neiva area): a record of climate, eustasy and crustal uplift during the Quaternary. Quat Int, 328-329: 94–106
Matte & Ribeiro, A. (1975). Forme et orientation de l'ellipsoïde de déformation dans la virgation hercyenne de Galice. Relations avec le plissement et hypothèses sur la genèse de l'árc ibéro-armoricain. C. R. Acad. Sc. Paris, 280.

Dias, R. & Ribeiro, A. (1995). The Ibero-Armorican Arc: A collision effect against an irregular continent? Tectonophysics, 246,113-128.

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